Of laminates obtained by extrusion lamination processing, a laminate comprising a polyethylene resin layer as at least one layer is used in wide use fields such as kraft package, flexible package, photographic support papers, tapes or various packages. Conventionally, a polyethylene resin used in those laminates has mainly been a branched low density polyethylene (hereinafter referred to as “LDPE” for brevity) due to its excellent molding processability. However, the density of LDPE is generally 0.918-0.925 g/cm3, and it is difficult to change physical properties that change together with the density, such as beat resistance, rigidity or gas barrier property. Thus, there has been restriction in using LDPE. On the other hand, a linear polyethylene such as a linear low density polyethylene (hereinafter referred to as “L-LDPE” for brevity”) or a linear high density polyethylene (hereinafter referred to as “LDPE” for brevity) can widely change its density according to the number of short chain branches. However, due to poor molding processability, it was difficult to obtain a laminate by extrusion laminate processing. In view of this circumstance, a method of obtaining a laminate by extrusion lamination processing a mixture of a linear polyethylene and LDPE was frequently used, as described in, for example, JP 6-65443A, JP 6-322189A, JP 7-92610A or JP 2000-73018A. However, this method gave rise to the problems on stability of product quality that because a mixing step of a polyethylene resin is required, cost performance of a laminate is poor; thermal deterioration causes in the mixing step of a polyethylene resin, and as a result, odor is liable to generate; and due to poor mixing of polymers, surface appearance of a laminate is poor. In particular, where a laminate is required to have heat resistance, rigidity, gas barrier property and easy disaggregation property, use of a polyethylene having high density, i.e., HDPE, is desired. However, LDPE must be mixed in order to improve molding processability, and this makes the density of the resulting mixture low, compared with HDPE alone. As a result, it was difficult to obtain a laminate having excellent heat resistance, rigidity, gas barrier property and easy disaggregation property. The term “easy disaggregation property” used herein means the property that a laminate having a paper as one layer is finely pulverized in a pulper. This property is strongly demanded in a moisture-proof paper having a resin such as polyethylene coated thereon.
Technologies are reported that long chain branches are introduced into L-LDPE or HDPE by changing a polymerization catalyst of ethylene, thereby increasing molding processability. However, melt tension of polyethylene resins obtained using those technologies are still insufficient, and LDPE had to be mixed, as disclosed in, for example, U.S. Pat. No. 5,272,236 and J. P. Hogan, et al., SPE J., vol. 23, p. 87 (1967).